The major theme to this research proposal will be to elucidate the mode of interaction of the blood (coagulation) proteins, Factor V(Va) and Factor X(Xa), plus calcium ions and phospholipids to form the lipoprotein enzyme complex, prothrombinase. An additional component of this study will be an attempt to delineate the mechanisms of attack of this prothrombinase on prothrombin (Factor II) to yield thrombin. Specific experimental procedures will include an examination of the conversion of Factor V, a plasma protein with a molecular weight of 430,000, to Factor Va a protein with a molecular weight near 200,000 by reagents such as Factor Xa (activated Factor X), Russell's viper venom activator (RVV-V) and thrombin. In addition to a chemical characterization of Factor V and Va (i.e. carbohydrate, lipid, amino acids, N-terminus, etc.), it is hoped to be able to define the role of this factor in the prothrombinase complex, and whether it may simply act as a large molecular weight cofactor. Additional studies will center on the role and importance of phospholipid binding to prothrombin and to Factors V and Va in the ultimate conversion of prothrombin to thrombin by the lipoprotein prothrombinase complex. This experimental approach is considered of prime importance to future studies aimed at understanding blood coagulation in vivo and specifically the behavior of platelets in this reaction.